The present invention relates generally to configurations of permanent magnets on a rotating member of a magnetic or superconducting bearing useful for flywheel energy storage and other rotating machinery that incorporate superconducting bearings. More particularly, the present invention relates to magnetic bearing designs encompassing a combination of permanent magnets and high-temperature superconductors (HTSs) which are capable of large levitation pressure, low eddy current loss, and good mechanical integrity at high speeds.
When these bearings are used in flywheel energy storage devices, the efficiency of the flywheel can be very high and flywheels of this type become economic for diurnal energy storage and other applications where high energy efficiency is important. Storage of electrical energy is useful in a number of applications. Diurnal storage of electricity is important to electric utilities in order to efficiently utilize base load generating plants and to meet the varying load demands of their customers. In this example, the base load plants can charge the storage units at night when demand is low, and then peak demands can be met by discharging the storage during the daytime peak hours. Energy storage can also play a substantial role in eliminating or postponing the installation of power lines with larger capacity. Power can be transmitted at night to a substation or user energy storage unit when demand is low, and then during peak power times, the energy storage units can be discharged. The placement of energy storage units can occur in various parts of the electrical distribution system: utility parks where large amounts of energy are stored; in tandem with photovoltaic or wind energy generation facilities that are time dependent; substation units; individual companies and houses. The invention can also be used for energy storage on electric vehicles such as cars and buses, or as wayside energy storage for electric trains or other transit vehicles.
Flywheels are often considered for energy storage applications. Their primary advantages are modularity, mechanical simplicity (low cost), high energy storage density (Wh/kg), and high efficiency input and output of electrical energy. The ability to produce high strength flywheel rotors and the ability to efficiently transfer energy in and out of a flywheel are well known and will not be discussed in this disclosure.
The primary disadvantage of conventional flywheels is inefficiency in standby storage mode. These losses occur because the bearings that support the flywheel structure have high losses. The present invention provides bearings having a very low rotational loss and can enable standby losses in flywheels to be 0.1%/hr or less. HTS bearings provide passive stability in all directions; i.e., they provide a positive stiffness in all displacement directions. Further, they allow rotational motion with very low friction.
A preferred HTS bearing embodiment of the invention uses permanent magnets, such as FeBNd or SmCo, on the rotor. For a large bearing, a permanent magnet ring or series of concentric rings is levitated over HTS elements, as shown in FIG. 1A. In order to maximize the levitation pressure, it is desirable to connect two or more magnet rings with a low-reluctance magnetic flux rerun path on top of the rings, as shown in FIG. 1A. This can be accomplished with a material of high magnetic permeability, such as Permendur brand high magnetic permeability material or magnetic steel.
One of the difficulties with HTS bearings is that for a large bearing, individual pellets or tiles of HTS must be arranged under the permanent magnet on the rotor. It is not yet possible to grow large, high-quality HTSs as one piece. Each of the HTS tiles will have its own magnetic field associated with it and present to the rotating permanent magnet and rotor an equivalent alternating current magnetic field. This can cause eddy currents in any conductor that is rotating with the rotor. In a first experiment, a permanent magnet ring was rotated with an aluminum rotor attached to it. A plastic rotor was attached to the permanent magnet ring in a second experiment. At the same levitation height, the rotational drag with the aluminum rotor was about an order of magnitude higher than with the plastic rotor. The results are shown in FIGS. 2 and 3. These experiments verify the importance of minimizing the eddy currents produced by the HTS tiles in the rotor to achieve a low-friction bearing.
In the preferred embodiments of the invention, the permanent magnets should have good mechanical integrity. Because these permanent magnets tend to have low mechanical strength, the present invention includes banding the magnets at intermediate locations so that they do not break apart due to the high centrifugal forces that they experience at high rotational speeds. Further, as noted previously, eddy currents can cause substantial rotational losses which can be lessened by selecting suitable banding materials.
It is therefore an object of the invention to provide an improved low-loss bearing and method of use that can be used to achieve a high-efficiency flywheel energy storage device.
It is a further object of the invention to provide a novel configuration and method of use of permanent magnets on the rotor that minimizes the eddy currents produced from the magnetic fields in an array of HTS tiles.
It is another object of the invention to provide an improved permanent magnet configuration and method of use that produces high levitation force.
It is yet another object of the invention to provide a novel design and method of use in which the permanent magnets can be spun at high rotational speed and maintain mechanical integrity.
It is a further object of the invention to provide an improved apparatus and method for reducing rotational losses and increasing structural integrity of rotating structures in superconductor applications.
It is another object of the invention to provide a novel banding configuration and method of use which reduces production costs by requiring only simple cutting operations to yield a structure capable of withstanding high centrifugal forces.
It is yet another object of the invention to provide an improved permanent magnet configuration for a superconducting bearing using an alternating polarity magnet configuration yielding balanced magnetic flux between individual components of the configuration.
Other advantages and features of the invention, together with the organization and the manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, wherein like elements have like numerals throughout the drawings.